The invention concerns magnetic shielding for a superconducting magnet coil arranged in the inside of a cryostat and having a vertical z-axis for generating a static magnetic field of a homogeneity of &lt;10.sup.-7 in the test volume of a nuclear magnetic resonance (NMR) spectrometer, wherein the shielding comprises spatial shielding at a distance from the cryostat in the form of a ferromagnetic shielding plate which is disposed above the cryostat.
Magnetic shielding of this type is known from DE 44 15 847 C.
The problems inherent in magnetic spatial shielding in the field of analytic NMR are described in detail in the above-mentioned document DE 44 15 847 C1. From U.S. Pat. No. 5,220,302 cited therein, "stave-like" magnetic shielding of a high field NMR magnet is known, in which the shielding elements surround the cryostat of the apparatus in the form of strip-like segments of a spherical shell at a distance symmetrically with respect to the central measuring volume.
The axial end areas are not shielded in this case. Consequently, the produced magnetic stray field can be shielded to the side, however, it emerges towards the top in an almost unhindered manner, such that, in a space located above the NMR spectrometer, it can fully develop its disturbing effect on sensitive devices.
The company leaflet "Introducing new technology for very high field NMR-spectrometers" from the Houston Advanced Research Center, 4800 research forest drive, The Woodlands, Tex. 77381, USA from the year 1993, discloses an integrated iron shielding of an NMR spectrometer, which is cooled down to the temperature of liquid helium.
In the field of NMR tomography, e.g. the article "A Cylindrically Symmetric Magnetic Shield for a Large-Bore 3.0 Tesla Magnet" by Ewing et al., MRM 29, Pages 398-401, 1993, discloses shielding chambers which are closed on all sides or are symmetrical. In such closed and symmetrical tomography shieldings, the NMR magnet coil is in the centre of symmetry of the whole arrangement.
Also from the field of NMR tomography, magnetic spatial shielding has become known through EP 0 182 284 B1 in which the thickness of the shielding can be varied and thus locally optimized. From the drawings shown in the document, it can be gathered that the shielding chamber consists of walls which are parallel to the axis, i.e. either of two side walls or a pipe which is arranged around the magnet and comprises either a circular cross-section or an n-fold, polygonal cross-section, or of a box-shaped configuration which has rectangular openings in the axial direction and has a square or coffin-like shape.
EP 0 196 511 B1 describes ferromagnetic shielding for NMR magnets, wherein the axial opening of an iron shielding cylinder, which directly surrounds the superconducting magnet, is calculated such that the field distortion in the test volume is as small as possible. In this manner, "retrofitting" of originally not shielded systems is to be realized.
U.S. Pat. No. 4,635,017 again shows a shielding arrangement for an NMR tomography magnet, wherein in this case "wings" are provided which protrude in the axial or radial direction from the magnet and are designed to capture part of the magnetic stray field.
Also from the field of NMR tomography there is finally known through DE 31 23 493 A1 ferromagnetic shielding which surrounds the magnet system in a relatively compact manner, in which variants with magnet coils having a vertical z-axis and shielding means having a horizontal or slightly curved "lid" are provided. Improved field homogeneity in the upper area of the test volume is regarded as an advantage of the curved "lid" wherein, however, in this type of tomography arrangement, the achievable magnetic field homogeneity in the measuring volume is by far worse than the homogeneity required for an NMR spectrometer. Furthermore, this does not concern spatial shielding at a distance from the magnet system but compact shielding which surrounds a normally conducting magnet coil system so that the problems with the use of a cryostat for superconducting magnet coils do not occur in this case either.
In contrast thereto, the initially cited DE 44 15 847 C1 describes magnetic spatial shielding for a superconducting NMR analytic magnet, wherein the ferromagnetic shielding is spaced away from the cryostat and contains at least two horizontally arranged plates, one of which is arranged below and the other above the cryostat. Moreover, vertical ferromagnetic side elements are provided which magnetically connect the upper and the lower plate. In this manner, shielding is effected which is considerably improved with respect to all other shielding devices for NMR analytic magnets hitherto known, wherein no major problems with respect to the action of forces nor significant disturbances of the homogeneity of the generated static magnetic field occur.
A disadvantage of this arrangement is still the fact that a considerable part of the magnetic stray field can leave the NMR spectrometer and enter into spaces located above the spectrometer. Even the application of a canopy-like surrounding "apron", provided in DE 44 15 847 C1, still allows that a very large amount of magnetic flux enters towards the top through the shielding. Furthermore, there is also the disadvantage that the known device requires a considerable amount of ferromagnetic shielding material which can cause, on the one hand, weight problems and problems with the statics of the building and, on the other hand, requires great expenditure in connection with the assembly.
Thus, it is the object of the present invention to improve magnetic spatial shielding having the initially mentioned features in such a manner that the magnetic flux in a room located above the spectrometer is minimized, if possible, completely eliminated and that the expenditure in connection with material and assembly for the shielding elements is considerably reduced.